Protein kinase CK2 is a heterotetrameric enzyme composed of two catalytic (CK2alpha) and two regulatory (CK2beta) subunits. This so-called "CK2-holoenzyme" is built from one stable CK2beta-dimer and from two CK2alpha- chains, which are bound to CK2beta-dimer and do not make direct contact with each other. In isolated state CK2alpha exists as a monomer. In contrast CK2beta is vailable as a dimer. CK2alpha belongs to the CMGC group of the eucaryotic protein kinases. Its nearest neighbours are cyclin �dependent kinases (CDK) and the mitogen-activated kinases. Homologous proteins for CK2beta are not established. CK2 shows the dual-co-substrate specifity, i.e. this enzyme can use ATP or GTP as phosphoryl donor. Protein kinase CK2 is a highly conserved protein kinase ever found in all eukaryotic cells investigated so far. The enzyme is important for cell proliferation and biomedically relevant because an overexpression of its catalytic subunit can cause lymphoma. The co-crystallyzation of these proteins with a non-hydrolyzable analog of ATP AMPPNP as a co-substrate and a peptide substrate may help in the investigation of the mechanism of the substrate binding and provide the much better understanding of the dual co-substrate specificity. The present work describes the crystal structures of two proteins hsCK2alpha-deltaC und hsCK2alpha-deltaC-V66A/M163/L. In both cases the binary complexes were charakterised structural with AMPPNP. The crystals of hsCK2alpha-deltaC belongs to the monoclinic space group P21 and the crystal structure was refined up to 2.3 Å resolution. The most important finding from this crystal structure is the discovery of beta4/beta5-loop in closed conformation form in contrast to the open conformation form observed for the CK2alpha subunits of the CK2 holoenzyme. CK2alpha monomers with this closed conformation are unable to bind CK2beta dimers because of sterical reasons. This finding confirms the growing evidence that CK2alpha monomers and CK2beta dimers can exist in vivo independently from the CK2 holoenzyme and may possess physiological role of their own. The solved crystal structure of the mutant hsCK2alpha-deltaC-V66A/M163L affords better understanding of the reason for the CK2 ability to use both ATP and GTP as phosphorylating agents. This mutant posesses the clear preference for ATP binding compared to GTP, which was confirmed by kinetic measurements. The crystalls of this protein belong to the tetragonal space group P43212. The best quality achived for this dataset was up to 1.6 Å resolution. The established conformational changes that occur upon AMPPNP binding comply with the reduced ability to use GTP. The thermostability of the CK2alpha, CK2beta and CK2 holoenzyme was examined by calormetric studies. Thereby the stabilizing effect of CK2beta on CK2alpha subunit was established. Moreover it was succeeded to determine the binding constants between the alpha and beta subunits in the CK2 holoenzyme by the isothermal titration calorimetry and differential scanning calorimetry. The values of the binding constant were found to be about 10^8 M^-1. Such values point to a non-obligate and transient komplex and comply with the former solved crystal structure of